The present invention is directed to lightning protectors, and more particularly to a system for protecting people, structures, and other assets from the hazards associated with lightning.
Lightning protection is needed wherever people or assets are perceived to be threatened by direct lightning strikes. Beginning with the most important one, such assets include people working in open areas, government buildings, conventional and nuclear power plants, military (Army, Navy, and Air Force) and para-military installations, radio and television stations, military and non-military launch sites (rocket installations under military, commercial, NASA, or other control), observatories and satellite tracking stations, churches, storage sites for flammable and/or explosive chemicals, public gathering places (for example open-air stadiums, golf courses), power transmission lines, etc. It is implied and understood that similar needs exist around the globe.
The primary objective of existing lightning protection is to deflect lightning away from structures, people, and assets. This is accomplished in prior art by the installation of one or more grounded lightning rods on tall towers, or on the highest points on buildings, which serve as lightning attractors. The method is based on a false notion that lightning always strikes the highest point in the area, and on the perceived existence of lightning protection zones (LPZ), which are described in many documents, including a standard Lightning Protection Code in the NFPA #78 document, issued by the (U.S.) National Fire Protection Association. Without the benefit of theory, NFPA #78 had been revised several times, to reflect the next best guess of the size and shape for the protection zone.
With safety as a primary objective, a plurality of grounding cables are anchored some distance away from a dielectric support structure underneath the lightning rod in one prior art lightning protection system. This configuration is intended to accomplish two things: (1) to conduct the intercepted lightning current away from work areas, and (2) to cover as many objects under the resulting umbrella-like structure as possible. For example, such a lightning protection system was designed and built by J. R. Stahmann in 1969 for the Navy 2D2 Site in Pinecastle, Fla. This protection system contemplates intercepted lightning currents being divided over a number of grounding wires before being injected into the ground at remote locations for significantly mitigating surface currents and the generation of hazardous step potentials in the work area below the lightning rod. A disadvantage of the multiple-grounded umbrella-like structure is that in fact, the volume of the lightning protection zone is decreased as the grounding wires are spread out. This means that instead of providing more protection to objects beneath the structure, the effect is quite the opposite! It can be shown, for example, that for a two-wire catenoid protection system, an angle of 60 degrees with the vertical eliminates the advantage of using two grounding wires as a way of extending the protection zone beyond that of the single-wire lightning protection system.
U.S. Pat. Nos. 4,180,698 and 5,043,527 to Carpenter, Jr. each disclose yet another variation of the lightning rod. The Carpenter, Jr. patents propose atmospheric ionizers as effective lightning eliminators. A metal brush-like structure is mounted on top of the lightning rod to serve as a source of ions to neutralize charge centers in clouds or elsewhere in the atmosphere. Although the method of discharging a charged body by mounting a sharp needle on it, by giving it sharp edges, or by bringing a grounded sharp needle near the charged body was already discovered some 250 years ago by the original inventor of the lightning rod, Benjamin Franklin (1747), it can be shown that the introduction of atmospheric ionizers may actually aggravate the lightning hazard, rather than eliminate it.
Thus lightning protection devices of the prior art are often ineffective both in preventing direct strikes on structures to be protected and in avoiding harmful surface currents and induced step potentials within nearby work areas. Further, prior art lightning protection devices are not designed to protect sensitive electronic equipment from damage by re-radiated EMI and RFI from down conductors.
Thus there is a need for a lightning protector that will mitigate or eliminate one or more of the following in a single structure:
1. Direct lightning strikes to protected structures; PA1 2. Surface currents and induced step potentials in work areas; and PA1 3. Re-radiated EMI and RFI from down conductors. PA1 (a) determining a minimum height .zeta. from which a lightning strike would likely be damaging to the secondary structure; PA1 (b) determining an ellipsoidal lethal zone (22) shaped according to the relation, EQU (h/z.sub.x.sup.- 1).sup.2 +(.rho./.rho..sub.x).sup.2 =1 PA1 (c) providing a conductive tower (10), the tower 10 having a height h.sub.o that is not less than z.sub.x ; PA1 (d) determining a cusped cone of protection (20) as a solid of revolution about the tower 10 when the tower is vertically projecting above level ground, by revolving the lethal zone about the tower, the lethal zone being tangent to the ground 12, touching the tower 10 at a height z.sub.x above the ground; PA1 (e) anchoring the tower 10 to the ground 12 proximate the secondary structure 18' at a location such that the structure 18' extends only within the protection zone 20; and PA1 (f) electrically grounding the tower 10 to the reference ground.